The advent of numerous private and commercial data storage and retrieval systems, commonly called hosts, has allowed persons with computers and modems to access information which would otherwise be unavailable or laborious to obtain. However, a single host frequently does not contain all of the information which a person may desire. Therefore, the person may have to access two or more hosts to obtain the desired information. In some cases, it may be necessary to repeatedly cross reference from one host to another host in order to obtain the desired information. Furthermore, after locating the desired information, the person may wish to transfer part or all of a data file from one host to another host in order to consolidate the information or update the second host. In the past, it has been necessary to terminate communications with the first host, dial up the second host, obtain the information, terminate the communication with the second host, dial up the first (or a third) host, obtain more information, etc. This is clearly a time consuming and inefficient procedure.
Data network services already exist which provide the user with access to more than one host, and CCITT Recommendations X.25 and V.42 and the LAP-M protocol provide a format whereby data is sent in the form of structured packets. A call request packet specifies the virtual circuit number. Once the virtual circuit (also sometimes referred to as a virtual channel) has been established the data is routed to a particular host or device by use of a virtual circuit number which is contained in the data packet. More and more, modem users are finding they can use packet-switched networks as a cost effective alternative to conventional long distance telephone calls. By connecting to a packet-switched network via a local phone call, a user can access national and even international networks. Packet-switching technology increases the overall system reliability since the network can accommodate a multitude of users by routing data in the most efficient manner and by bypassing equipment which might be out of service.
Therefore, once the user gains access to one of these networks, usually via a dial-up connection and a modem, the user may obtain information from selected ones of the hosts by sending a first query addressed to a first selected host, a second query addressed to a second selected host, etc. However, since the user accesses the network through a single modem, if the DTE does not have X.25 or LAP-M capability then the modem must have X.25 or LAP-M capability and, furthermore, the user must have a way of telling the modem that a particular query or piece of data is intended for a particular host.
Therefore, there is a need for a method whereby a user can specify or ascertain that a query or set of data is intended for or came from a particular host.
There is also a need for a method whereby data terminal equipment (DTE), such as a computer, can instruct data communication equipment (DCE), such as a modem, that a particular set of data is destined for a particular host in an attached network.
There is also a need for a method whereby data communication equipment can instruct data terminal equipment that a particular set of data has originated from a particular host in an attached network.
There is also a need for a method whereby a simple device can simultaneously communicate with two or more other devices over a plurality of virtual circuits.
Also, the different hosts may specify different compression schemes, encoding schemes, or other criteria. If only one PAD (Packet Assembler-Disassembler) is used then, to switch from one host to the next, the user will also have to reconfigure the PAD. Therefore, there is a need for an apparatus which has a plurality of independent PADs.
Generally, communications and control functions between a DTE and a DCE are accomplished by using the transmit data and receive data conductors, in conjunction with other conductors such as the data terminal ready (DTR) conductor, the ring indication (RI) conductor, the data set ready (DSR) conductor, etc. However, the usage of and particular function accomplished by these other conductors varies from device to device and from manufacturer to manufacturer. Therefore, there is a need for a communication and control protocol which can be effected using only the transmit data and receive data conductors.
Since multiple channels are to be transferred over a single data link, the transfer of a break signal, in its normal form, can noticeably reduce the effective data transfer rate for all channels. The effective data transfer rate could be improved if, rather than transferring the break signal in its original form, an indication of the break signal was transferred instead. In some protocols, the length of the break signal is of significance. Therefore, there is a need for a protocol which can provide an indication that a break signal has occurred and an indication of the length of the break signal.